Field of the Invention
The present invention relates to toners, image forming apparatuses, and process cartridges.
Description of the Related Art
Conventionally, an image forming apparatus employing an electrophotography or electrostatic recording manner has been known to develop a latent image with a toner to form an image. In recent years, a low temperature fixed toner capable to be fixed at a lower temperature has been being developed from the viewpoints of enhanced environment consciousness and a lower running cost.
A unit configured to fix the toner usually employs a manner in which a heat-melted toner is pressured. In this manner, in order to fix the toner at a low temperature, the toner is required to be melted at a lower temperature. However, when a melting temperature of the toner is too low, the toner is prematurely softened before fixing, resulting in failure regarding to toner replenishment or image quality. In order to eliminate such trade-off, the toner is basically designed to melt sharply so as to give steep slope of change of melt viscosity against temperature.
To achieve this, it has been examined to decrease a molecular weight and narrow a molecular weight distribution of a binder resin contained in a toner as possible. Japanese Patent Application Laid-Open (JP-A) No. 2013-137420 discloses an attempt to use a plasticizer or a crystalline material. Specifically, JP-A No. 2013-137420 discloses an attempt to control an endothermic peak temperature and a molecular weight of a crystalline polyester to thereby improve a low temperature fixing property and heat resistant storability,
The binder resin often contains oligomers, and monomers or hydrolysate remaining after synthesis as low molecular weight components having the molecular weight of 1,000 or less. For example, polyester commonly used in the toner contains various alcohols, carboxylic acids, and low molecular weight esters because an esterification reaction for synthesizing the polyester is an equilibrium and reversible reaction.
The low molecular weight components contribute to low temperature fixing owing to its low molecular weight, but are likely to absorb moisture in an atmosphere owing to the presence of polarity groups (e.g., carboxyl group and hydroxyl group) in a relatively high ratio. Therefore, when the toner is stored or used under a high temperature and high humidity environment such as the rainy season in Japan, the toner is deteriorated in charging ability over time. In particular, as the binder resin is decreased in molecular weight in order to achieve the low temperature fixing, a percentage of the low molecular weight components is increased relative to the total resin. As a result, it has been known that the toner is greatly deteriorated in charging ability, leading to unstable developing ability of the toner and poor image quality.
In order to solve the above problem, JP-A No. 2008-015230 discloses an attempt that an acid value and a hydroxyl value of a certain toner material is decreased to a predetermined value or lower. A relationship between a flowing-out beginning temperature of the toner and a melting point of the crystalline polyester is also examined to improve the low temperature fixing property and prevent reduction in image density under the high temperature and high humidity environment.
However, in view of the low temperature fixing property of the toner, the acid value and the hydroxyl value are believed to have a suitable range. Therefore, it is inappropriate to decrease the values too much. In JP-A No. 2008-015230, the acid value and the hydroxyl value are defined for only the certain toner material. However, chargeability of the material is affected by its surface-exposed amount on toner particles, and, in turn, the surface-exposed amount varies with each production step, in particular, toner forming step, so that an expected effect cannot be achieved in some cases, which is problematic.
From the above, reduction of a charging amount of the toner affected by temperature and humidity is problematic in designing a toner having satisfactory low temperature fixing property which has been recently demanded. Therefore, there is a need for achieving both a good low temperature fixing property and a reduced change of charging amount. Further, there is a need for a toner having excellent hot offset resistance and storability while achieving both a good low temperature fixing property and a reduced change of charging amount affected by temperature and humidity.